1. Technical Field
This disclosure relates to catalyst systems for olefin polymerization. More specifically, this disclosure relates to metallocene catalyst systems and polymers produced therefrom.
2. Background
Olefin polymers and copolymers such as polyethylene, polypropylene and ethylene-propylene can be produced under various polymerization conditions and employing various polymerization catalysts. In the case of C3 or greater alpha olefins, the resulting polymer may exhibit stereoregularity. For example, in the case of propylene, a polypropylene product may be isotactic wherein each methyl group attached to the tertiary carbon atoms of the successive monomeric unit falls on the same side of a hypothetical plane through the main chain of the polymer. Polypropylene may also be syndiotactic wherein the methyl groups attached to the tertiary carbon atoms of the successive monomeric unit are arranged as racemic dyads. In other words, the methyl groups in isotactic polypropylene lie on the same side of the polymer backbone whereas in syndiotactic polypropylene the methyl groups lie on alternate sides of the polymer backbone. In the absence of any regular arrangement of the methyl groups with respect to the polymer backbone the polymer is atactic. The stereoregularity of the polymeric product impacts both the physical and mechanical properties of said product.
Fluorenyl-type metallocene catalysts are effective catalysts in the polymerization of olefin polymers such as ethylene, propylene and higher olefins or other ethylenically unsaturated monomers into homopolymers or copolymers. Fluorenyl-type metallocenes are generally characterized by bridged cyclopentadienyl and fluorenyl groups that serve as a ligand to a metal atom. Varying the substituents or position of substituents on the fluorenyl group, cyclopentadienyl group or bridging moiety of a given fluorenyl-type metallocene catalyst may produce polymers having very different physical properties. For example, an isomer of a fluorenyl-type metallocene catalyst may produce isotactic polypropylene, while another isomer of the catalyst may produce syndiotactic polypropylene. In addition, properties such as the molecular weight and melting points of the polypropylene composition may vary and as a result the mechanical properties and utility of the polymer may vary.
Thus, there is an ongoing need for catalysts capable of producing stereoregular polypropylene compositions with differing physical properties such as molecular weight and melting points.